1. Field of the Invention
The present invention generally relates to a passive component, and more particularly to a choke coil.
2. Description of the Prior Art
Referring to FIG. 1A and FIG. 1B, conventional choke coil 100 includes a drum core 110, a coil 120, and a shell 130. The drum core 110 includes a middle core 112, an upper core 111, and a lower core 113. The upper core 111 and the lower core 113 are connected to opposing ends of the middle core 112. The coil 120 is wrapped around the drum core 110. The shell 130 surrounds the coil 120 and the drum core 110. Moreover, there is an air space t between the coil 120 and the shell 130. There is also an air space t between the drum core 110 and the shell 130.
When the drum core 110 is disposed in the center of the conventional choke coil 100, the inductance of the conventional choke coil 100 is about 4.45 uH. When the drum core 110 is shifted and touches the shell 130 as shown in FIG. 1C, the inductance of the conventional choke coil 100 is about 6.44 uH. As the position of the drum core 110 changes, the air space t changes, and, as a result, the inductance of the conventional choke coil 100 also changes.
Therefore, during the manufacturing process of the conventional choke coil 100, the drum core 110 should be precisely positioned so as to fix the air space t to ensure that the conventional choke coil 100 has a constant inductance for different instances of the conventional choke coil 100. However, the process of precisely positioning the drum core 110 increases the cost of manufacturing the conventional choke coil 100. Moreover, the air space t decreases the magnetic flux passing through the drum core 110 and the shell 130, and, as a result, decreases the inductance of the conventional choke coil 100. The inductance of the conventional choke coil 100 is able to be adjusted by changing the number of turns of the coil 120 and the dimension of the drum core 110.
Another conventional choke coil (compression molding type) is shown in U.S. Pat. No. 6,204,744. A coil and a powder magnetic material are placed within a mold cavity of a pressure molding machine, and then the choke coil is formed by applying a high pressure. Because the coil is not sufficiently supported within the pressure molding machine, the insulating coating of the coil may be removed due to the pressure of the forming process. As a result, the choke coil may have the problem that the coil is shorted.